1. Field of the Invention
The present invention relates to a method for connecting a metal workpiece and a plastic workpiece by means of a joining tool.
2. Discussion of the Prior Art
In the fields of automotive and aviation technology as well as in medical applications devices are often employed which are a combination of metal and plastic workpieces, and it is expected that the number of these applications will significantly grow in the future. Such combinations are often used in order to reduce weight by still maintaining the desired properties of the device, e.g. stiffness or resistance against corrosion. In this regard, it is required to provide for reliable methods to join the different components of the devices and in particular to join metal components with plastic components in a sufficiently reliable and efficient manner.
From the prior art it is known to employ rivets or screws for the purpose of connecting workpieces of different materials. However, these mechanical connections involve additional elements which lead to an increase of the weight of the device. In particular, for aviation applications, this effect is disadvantageous as it is a main requirement in this field to ensure that the weight of a each device is minimized. Further disadvantages of mechanical connections are the fact that high stress concentration levels are generated at the location of the connection, and that the connection itself may form the starting point for a crack in at least one of the components. Finally, a connection with an additional element such as a screw or rivet may lead to an unpleasant appearance which is an often unwanted effect.
Whereas in case of connections between two metal components conventional welding techniques such as ultrasonic, resistance, vibration, infrared or induction welding, have proven to be extremely useful even in situations where only punctual connections are possible, these techniques cannot be employed in case of metal/plastic connections in the same manner as in metal/metal connections. Usually the welding temperatures for metal are much higher than for thermoplastic materials, and thermoset materials cannot be welded at all since these materials do not melt. Therefore, the weld seam has to be positioned in the thermoplastic material in its entirety whereas the structure of the metal component remains essentially unchanged.
Further, it is disadvantageous that when traditional welding techniques are applied, often a previous surface treatment is required which is time consuming. Moreover, welding is often connected with a high energy input, which may lead to the problem that the material in the vicinity of the welding point is significantly influenced and the material properties are altered.
Another viable option is the application of special adhesives to connect workpieces formed of different materials. Here, it has turned out that those adhesives which provide for a sufficiently stable connection, produce toxic gases at least during curing or require the application of heat so that additional measures are necessary to prevent the gases from getting into contact with workers in the vicinity of the connection formed by gluing or the materials from being altered due to the heat.
From U.S. Pat. No. 7,497,917 it is know to produce a connection between metal and plastic workpieces by providing a layer of heat activatable adhesive on the surface of the plastic workpiece that is facing towards the metal workpiece. Subsequently, a friction stir welding tool engages with a surface of the metal workpiece opposite of the plastic workpiece. Due to the frictional heat generated by the rotatingly driven friction stir welding tool the adhesive is molten. After the tool has been stopped to rotate and removed from the surface of the metal workpiece the adhesive will harden and a joint will be formed.
Moreover, it is described in U.S. Pat. No. 7,497,917 to connect a metal workpiece and a plastic workpiece by first forming a hole in the metal workpiece and then bringing the metal workpiece in contact with the plastic workpiece. In the following, a ring shaped friction stir welding tool is brought into engagement with the surface of the metal workpiece so that it surrounds the hole. Due to the heat generated by the rotating friction stir welding tool the plastic workpiece melts and plasticized material flows into the hole so that a positive joint with mechanical interlocking between the joining partners is formed.
The methods described in the afore-mentioned publication suffer from the following disadvantages. In the first embodiment, it is required to apply the adhesive to one of the workpieces, so that in this case an additional step is needed which makes the method time consuming and less flexible. Further, it is questionable whether a joint formed by an adhesive only is sufficiently stable. In the second example it is required that in a first step a hole is drilled into the metal workpiece which is an additional time consuming step. Further, plastic material flows into the hole and to the visible side of the metal workpiece. Thus, this might lead to the unwanted effect that the position of the joint can easily be identified. Finally, a hole has to be produced in the metal workpiece which leads to a reduced strength of the workpiece in the region of the joint.